1. Field of the Invention
The present invention relates to an improved process for desulfurizing and refining hydrocarbon fractions containing large quantities of aromatic components.
2. State of the Art
Aromatic hydrocarbons such as benzene, toluene and xylenes (hereinafter referred to collectively as "BTX") are contained in large quantities in, for example, cracked gasoline, by-product of thermal cracking of naphtha. The aromatic raw materials such as the cracked gasoline contain organic sulfur compounds as the impurities at a relatively high concentration as well, and therefore, it is necessary for effective utilization of BTX to desulfurize and refine the raw materials.
The desulfurization is usually performed by catalytic hydrotreating of the organic sulfur compounds so as to convert them to hydrogen sulfide, and then, removing the hydrogen sulfide by means of washing with alkaline aqueous solution.
Various types of catalysts are useful for the hydrotreating, and amoung them, Co--Mo system catalyst is the most popular one. The regulation on discharge of NO.sub.x, which is getting more strict in recent years, requires removal of organic nitrogen compounds in the raw material oil. For the purpose of simultaneous denitrification with the desulfurization, Ni--Mo system catalyst and Ni--Co--Mo system catalyst came to be used often because of their high performance of denitrification.
The aromatic raw materials such as the cracked gasoline generally contain polymerizable compounds like diolefins as well, which quickly polymerize at a temperature at which the hydrotreating of organic sulfur compounds is conducted to cause troubles of plugging of the reaction apparatus. So, it is advantageous to convert firstly the polymerizable compounds to non-polymerizable hydrocarbons by hydrogenating them at a relatively low temperature at which the polymerization does not proceed, and then, to convert the organic sulfur compounds to hydrogen sulfide by hydrotreating at a relatively high temperature. Thus, there has been established the two-step method as a practical process, which comprises the former step, hydrogenation of the polymerizable compounds using a palladium catalyst, and the latter step, hydrotreating of the organic sulfur compounds using a Co--Mo system catalyst, a Ni--Mo system catalyst or a Ni--Co--Mo system catalyst.
The problem awaiting solution in the hydrotreating of the organic sulfur compounds using the Co--Mo system, Ni--Mo system or Ni--Co--Mo system catalyst is, in addition to how to achieve a high rate desulfurization by perfectly converting the organic sulfur compounds to hydrogen sulfide, how to decrease loss of BTX, the aimed product, by minimizing hydrogenation thereto. In general, higher activity of the hydrogenation catalyst achieves the former object, but the latter intention is difficult to realize. In fact, according to the conventional technologies, in case of desulfurization of a raw material containing organic sulfur compounds of several hundreds of ppm to decrease residual sulfur compounds to as low as 1 ppm in terms of sulfur or less, loss of BTX by hydrogenation thereto of cyclohexane or its derivatives inevitably reaches 3 mol %, or even 5 mol %. This problem is particularly serious when Ni--Mo system catalyst or Ni--Co--Mo system catalyst is used. These catalysts are preferred in general for the objects of hydrodesulfurization and hydrodenitrification because they have higher performance of hydrogenation, but at the same time, they have the demerit that the loss of usefulcomponents, BTX, due to their hydrogenation performance occurs very easily, which is greater than in the case of Co--Mo system catalyst.